With the development of communication technology, demands increase especially in secure communication field. In theory, benefits of quantum communication having absolute security attract more and more attentions. Quantum Key Distribution (QKD) is the core of quantum cryptography technology, which may achieve unconditional secure key distribution in the sense of cryptography between remote users. It is possible to realize unconditional secure communication by combining quantum cryptography technology and one-time pad. Therefore, as a new direction of cryptography, the quantum cryptography technology has been widely concerned.
A quantum key distribution system's operating rate is an important factor for deciding its key generation rate. Higher key generation rate means an ability of providing security safeguards in a wider range of applications. Therefore, improving the operating rate of the quantum key distribution system is of important significance. BB84 protocol is the most widely used quantum key distribution protocol. The protocol requires to encode a photon into four states randomly, i.e., randomly modulating a phase, polarization and the like physical quantities of the photon into one of four quantum states in two non-orthogonal basis. In ordinary single-mode optical fiber, phase-encoding scheme is usually employed since intrinsic birefringence effect of optical fiber will change the polarization of the photon.
One key technology for realizing a high-speed QKD system lies in how to achieve random and high-speed encoding and decoding of quantum signals. For a high-speed phase modulating system, an electro-optic modulation device based on lithium niobate (LiNbO3) waveguide materials is usually used to realize phase modulation of a photon's phase, where a modulated phase is directly associated with a voltage driving signal loaded onto its driving electrode, and accuracy of the modulating voltage decides accuracy of the phase modulation. Due to a direct association between Quantum Bit Error Rate (QBER) of the QKD system and modulation accuracy of quantum state, a driving signal for phase modulation of the phase QKD system is demanding on signal to noise ratio and flatness. The existing method for generating a modulated quantum signal mainly includes digital to analog conversion, an analogue switch and the like methods. In a QKD system within 200 MHz, these methods usually meet speed and accuracy requirements for switching of modulating voltages. However, due to restrictions caused by establishment and stabilization time in the digital to analog conversion, switching time in the analogue switch and the like factors, it is difficult for these methods to meet requirements of modulation rates above 1 Gbps.